Statistical display apparatus

ABSTRACT

A statistical display apparatus for displaying information about the magnitude of a variable in histogram form comprises a number of capacitors corresponding to the number of classes into which the magnitude of the variable is to be classified. The variable is periodically sampled and, in accordance with the magnitude of the sample, an appropriate capacitor is selected by a steppingswitch unit and a unit charge is fed into the selected capacitor. In this selection, all the capacitors are scanned by the stepping switch and part of the charge is removed from each capacitor. The capacitors are also scanned in sequence and the magnitudes of their charges are displayed on a cathode-ray tube.

H11 ags Ronald William Taylor llltcliln, England Datrnn Limitedlliitchln, England STATlSTlCAL DISPLAY APPARATUS 12 Claims, 4 DrawingFigs.

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lFieltl oi Search Reierences Cited UNITED STATES PATENTS 12/1957 Bamothy==-ll STAGE CAPACITOR STURE Primary Examiner-- Donald J YusltoAttorney-Cushman, Darby & Cushman ABSTRACT: A statistical displayapparatus for displaying information about the magnitude of a variablein histogram form comprises a number of capacitors corresponding to thenumber of classes into which the magnitude of the variable is to beclassified. The variable is periodically sampled and, in accordance withthe magnitude of the sample, an appropriate capacitor is selected by astepping-switch unit and a unit 4 charge is fed into the selectedcapacitor. in this selection, all 24/1 340/324 A the capacitors arescanned by the stepping switch and part of lnt. Cl 601i 7/00, the chargeis removed from each capacitor. The capacitors are 1 31/08 also scannedin sequence and the magnitudes of their charges 340/169, are displayedon a cathode-ray tube. 172, 324

SWlTIIH CUllTRUL UNIT.

EUINEIDENCE CIRCUITS.

WENIEB me] am 3529, 353

SHEET 2 BF 3 STATlIFiTll CAL lDllSlPlLAif AlPlPAllTATlUS BACKGROUND OFTHE llNVENTlON 1. Field of the invention This invention relates toapparatus for the statistical display of a variable, for examplerandomly varying analogue quantity, and is directed more particularly toproviding a histogramtype display indicating the frequency with whichthe magnitude of the variable falls within predetermined classes, thatis to say between predetermined ranges of values, and in which thedisplay continuously updated.

2. Description of the Prior Art [n a conventional-type of histogramdisplay, a variable is periodically sampled and, over a period of time,a count is made of the number of times in which the sample value fallswithin the various classes. The various counts can be displayed so as togive a histogram-type display. It is necessary however, at the end ofthe counting period, to remove the information and restart counting. inthis type of system, the display information is only of significancetowards the end of a counting period and for many purposes it can beutilized only at the end of the period. The present invention however isdirected to a system in which the display is continuously updated by theremoval of stale information so that the display is continuously ofsignificance in indicating the required information.

An updated histogram display has been described in U.S. Pat. No.3,317,132 of R. E. Martin making use of a mechanical system using ballswhich are fed into tubes to build up a histogram display or of a digitalsystem using digital processing techniques.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide an improved form of statistical display apparatus for giving anupdated form of statistical display apparatus for giving an updatedhistogram display making use of analogue techniques enabling informationin histogram form to be available as analogue voltages, suitable forexample for displaying directly on a cathode ray tube.

According to this invention, a statistical display apparatus fordisplaying information about the magnitude of a variable comprises anumber of capacitors corresponding to the number of classes into whichthe magnitude of the variable is to be classified, means forperiodically sampling the variable and classifying its magnitude intothe appropriate one of the classes and for feeding a charge into thecapacitor corresponding to the appropriate class, means for periodicallyremoving part of the charge on each of the capacitors and display meansfor displaying the magnitude of the charges or mean charges on each ofthe various capacitors.

A charge may be introduced on to a capacitor either by connecting thecapacitor through a resistance to a source of charging potential for atime period short compared with the time constant of the chargingcircuit or by connecting the capacitor to an auxiliary capacitor whichhas previously been charged. For convenience in the followingdescription, the various capacitors corresponding to the differentclasses will be referred to as the storage capacitors as they form thedata stores. As will be explained later, under steady state conditions,if all the storage capacitors are of equal magnitude and if all thecharging and discharging circuits are the same or similar, the voltageon each of the storage capacitors is directly proportional to thefrequency with which the variable is classified into the appropriateclass. This form of apparatus however has the particular advantage thatunder changing conditions, the display is more closely representative ofrecent sampled information than of stale information. That this is somay readily be seen by considering a storage capacitor into which somecharge has been put in the past but where no recent sampled values fallwithin this class. it will be appreciated then that this capacitor willbe discharged exponentially, i.e. the rate of loss of charge is greaterinitially than it is later. The charge on the capacitor is thereforeless than would be the case if the sampled values in this class had beencoming in at a uniform rate corresponding to a true average frequencyover a period of time. if on the other hand recent samples are fallingfrequently in this class the charge will tend to build up exponentiallyand the charge will be higher than if the sampled values had beenreceived uniformly over a period of time at the actual average rate overthat period.

The display means may conveniently be a cathode-ray tube display, thevoltages on the various storage capacitors being scanned in sequence andutilized to effect transverse deflection of a line scan trace so therebysimulating a conventionaltype of histogram display. it will beappreciated that such a display on a cathode-ray tube can he obtained asfrequently as is required; in effect one can obtain a continuous displayin histogram form of the classification of the magnitude of the variablequantity. Other types of indicating apparatus however may be employed;for example the voltages on the various storage capacitors may beutilized to control a chart recorder which periodically records thesemagnitudes or the voltages may be processed electrically, for examplethey may be utilized in analogue or digital-computing apparatus.

in one simple form of the apparatus the various capacitors correspondingin number to the various classes are made of equal capacity. Each ofthese capacitors has one of its poles connected to a lead at a commondatum potential, for exam ple earth. The other poles of the variouscapacitors are connected to a common line via separate switches for eachof the various capacitors which switches are operated in sequence tocomplete the circuits for predetermined short durations, the variousswitches being operated in such a manner that only one capacitor at atime is connected to :said common line. For example, this common line isconnected to one end of a resistor the other end of which is connectedby a further switch enabling that end of the resistor to be connectedeither to a source of charging potential or left open circuit orconnected to a datum potential, conveniently earth. Preferably however aswitch unit is employed having separate switch contacts for completingcircuits as required through separate resistors to the source ofcharging potential or a discharge (e.g. zero potential) point. Thisswitch unit is operated in synchronism with the previously mentionedswitches but is controlled by the sampling and classifying means so thatthe charging circuit is completed when the particular switch associatedwith the particular capacitor corresponding to the class in which thevariable is found to fall is closed. The switch unit connects the commonline to the datum potential (usually earth) each time the switchassociated with a capacitor not corresponding to the appropriate classis closed. It will be seen therefore that when the variable isclassified, the appropriate capacitor is connected to be charged througha resistor to receive an additional charge whereas each of the othercapacitors will be partially discharged during the switch operationscanning sequence. In this arrangement the output to the display meansfrom the capacitors may conveniently be the voltage on theaforementioned common line. During the switch scanning sequence, thisline is connected to the various storage capacitors in succession andhence the switch scanning sequence provides the scanning of the outputvoltages.

in some cases however it may be preferred to have a separate resistorfor each capacitor as this allows the readout circuit to have adifferent sensitivity for each class. By such an arrangement thehistogram scale could be weighted according to the class number.

If the various storage capacitors are to be charged and discharged usingan auxiliary capacitor, it is convenient to use a single auxiliarycapacitor for both charging and discharging purposes. in such anarrangement, instead of using a resistor between said further switch andsaid common line, the further switch may be connected directly to saidcommon line and this common line may be connected to one pole of theauxiliary capacitor the other pole of which is connected to a resistorto the source of the datum potential. With this arrangement, whilst thevarious switches associated with the different capacitors are open, thefurther switch has to be closed and opened so as either to put a chargeon the auxiliary capacitor or to remove any charge therefrom accordingas to whether or not the next capacitor to be connected is to be chargedor discharged BRIEF DESCRIPTION OF THE DRAWINGS The following is adescription of a number of embodiments in the invention reference beingmade to the accompanying drawings, in which:

FIG. 1 is a diagram illustrating a form of statistical displayapparatus; and

FIGS. 2, 3 and 4 are diagrams for explaining modifications to thearrangement of FIG. 1. Referring to FIG. 1 there are shown a number ofcapacitors 10, 11, 12 etc. corresponding to the number of classes intowhich the variable is to be classified. Typically there are eleven suchcapacitors. One pole of each of these capacitors is connected to acommon line 19 which may conveniently be earthed. The other poles ofthese capacitors are connected respectively via switches 20, 21, 22 etc.to a second common line 29. These switches 20, 21 22 may be mechanicalswitches, e.g. reed switches or they may be electronic switches, e.g.transistors. Mechanical switches are preferred because of a higherinsulation when open circuit, preventing loss of charge on thecapacitors. These switches are operated in sequence for a period whichhas to be short compared with the time constant of the charging anddischarging circuit for the capacitors to be described subsequently. Thevarious switches are normally open and only one is closed at any onetime in a manner to be described later. The common line 29 is connectedto a charge/discharge switch unit 30 containing two resistors 31, 32.The resistor 31 is connected between the line 29 and a switch contact 33leading to a source 34 of direct voltage V whilst the resistor 32 isconnected between the line 29 and a switch contact 35 leading to a zeropotential source 36.

The switches 20, 21, 22 are controlled to operate in sequence by aswitch control unit which gives a switch operating output on a selectedone of a set of lines 41 when a signal is received from a timinggenerator 42, the particular output line 41 being selected by a stepper43. This stepper is also controlled by timing pulses from the timinggenerator 42 to step on one step in a cycle sequence for each outputsignal from the timing generator.

The input information to be displayed may typically be the outputsignals from a classifier 45 which classifies an analogue input voltageon a line 46 into a number of classes, each corresponding to a selectedrange of magnitudes of the input voltage and provides an output signalson a selected one of a series of lines 47 corresponding to theappropriate class. One construction of classifier suitable for thispurpose is described in the specification of copending U.S. ApplicationNo. 743,891 of R. W. Taylor entitled Apparatus for statisticallyclassifying an analogue voltage. The classifier 45 initiates thescanning cycles controlled by the timing generator 42 to effect thecyclic switching of the stepper 43 in synchronism with the sampling, onecycle of the operation of the stepper being effected for each sample.The output from the classifier 45 on the lines 47 is applied to a set ofcoincidence circuits 48 to which are fed outputs from the stepper 43.The coincidence circuits 48 give output signals on lines 49, 50 when thestepper 43 reaches the step corresponding to the output line 47 selectedby the classifier 45. The output on line 49 is applied to the switchunit 30 as a control pulse to effect selection of switch contact 33 inplace of switch contact 35. The selected one of the switch contacts 33or 35 is operated by a timing pulse on a lead 51 from the timinggenerator 42 in synchronism with the pulses applied to the control unit40 and the stepper 43.

The switch unit 30 is thus operated synchronously with the switches 20,21, 22 but is controlled by the classifier 45 so that the line 29 isconnected to the voltage source 34 when the particular one of theswitches 20, 21, 22 etc. corresponding to the appropriate class isclosed. The line 29 is connected to zero potential source 36 when any ofthe other switches 20, 21, 22 etc. are closed. A complete scan of allthe capacitors 10, 11, 12 is made for each sampling cycle, the switchunit 30 connecting the line 29 to the zero potential source for all theclasses except the selected one.

Usually the various capacitors 10, ll, 12 etc. will be made of equalmagnitude C. Assuming that the resistors 31, 32 are both of magnitude R,the charging voltage is V, and that the switches 20, 21, 22 etc. areclosed for a time, t a then, if steady state conditions have beenreached such that the average rate at which measurements are made in oneof the classes C,is n ,per unit time, the average capacitor voltage Vwill be defined by equating the currents flowing into and out ofappropriate capacitors, i.e.

where N is the measurement rate.

Hence V G V The voltage V is thus directly proportional to the frequencyat which the sampled values are entering the class C It will benotedthat the capacitor charge in each of the capacitors 10, 11, 12etc., if no charge is added, is removed exponentially by the repeatedconnection through the switch contact 35 to the zero potential source ineach switch scan. The removal of part of the voltage occurs at thestepwise scanning rate irrespective of the regularity of the scanning orof the measuring rate.

As is indicated diagrammatically in FIG. 1, a cathode-ray tube display37 is provided with a saw tooth signal applied to the X deflectionsystem from a time base generator 38 to give a scanning trace with issynchronized from the timing generator 42 to start at the start of thecyclic scanning sequence of the switches 20, 21, 22 etc. The voltage atthe common line 29 is applied, via a modulator 39, to the Y deflectionsystem so that the display trace is a histogram type display. Thismodulator consists essentially of a switch controlled by the timinggenerator 42 which switches either the output line 29 or earth to the Ydeflection system of the display 37. This modulator is arranged so thatthe output line 29 is connected to the display only during periods whenone of the switches 20, 21,22 etc. is closed, the display input beingearthed at all other times. The timing generator 42 thus controls thesequence of operations which occurs each time the variable is sampled.The sequence of operations is as follows:

1. Stepper 43 is moved to the next step and, on a pulse signal from thetiming generator 42, the switch control unit 40 energizes the selectedrelay coil, the output to the display being earthed by the modulator 39during this period.

2. The charge/discharge switch unit 30 is operated, the output line 29being connected either to the constant voltage source 34 via resistor 31or to zero potential source 36 via resistor 32 according as to theoutput giving by the coincidence circuits 48. The modulator 39 connectsthe output line to the display 37.

3. The coil of the selected switch 20, 21 etc. is deenergized and theoutput to the display earthed. Typically the time period for the firstof these operations to the commencement of the second operation would beabout I millisecond, for the second operation to the commencement of thethird about 8 milliseconds and for the third operation to the start ofthe next sequence about 1 millisecond giving a total time of about 10milliseconds for a complete scanning sequence. The-display is renewed ateach measurement and therefore provides a continuously updated relativefrequency histogram having the basic properties of a probabilityfunction.

Winn

It may be desired to indicate on the display the particular class inwhich an entry is being made. This is done by providing an output fromthe coincidence circuits Iii on the line 59 which is applied as abrightening pulse to the display when the stepper is in a positioncorresponding to the class being entered.

If it is required to have a normal histogram display showing thefrequency of entry into the various classes for a predetermined periodwithout any updating, this can be achieved by not discharging thecapacitors and by charging the selected one from a constant currentsource. FIG. 2 illustrates a modified circuit arrangement for the switchunit 30 suitable for this purpose. In FIG. 2, by putting a switchcontact 52 in its left'hand position, the circuit arrangement iseffectively that of FIG. 1, having resistors 31, 32, switch contacts335, 35, a constant voltage source 34 and a zero potential source as.When switch contact 52 is in its right-hand position, the line 29 can beconnected by a switch contact 53 to a constant current source 54 or, bya switch contact 55, through a resistance 56 to a zero potential point57. The switch 92 may be a manual switch set as required. For a normal(i.e. not updated) histogram display giving the frequency of entry intoclasses within a predetermined period, the switch 52 is put to theright-hand position, switch 55 is left open circuit and switch contact53 is operated by the coincidence circuits so as to be closed once ineach scanning cycle when the stepper 413 is at the positioncorresponding to the class to be entered. With a constant currentsource, the capacitors It), 11, 12 etc. are charged linearlyproportional to the number of entries. The accuracy in relation to theperiod over which measurements are made will depend on the quality ofthe storage capacitors 10, 11, 12 etc. and on the impedance of thereadout circuit. It would be possible to use the switch contact 52 inthe righthand position for an updated display by operating the switchcontact 55 for each step of the scanning to remove part of the charge onthe capacitors 10, 11, 12in a manner similar to the operation of switchcontact 35 of FIG. ll. In this case, however, since the capacitors arelinearly charged, it is desirable to remove part of the charge fromevery capacitor during the cycle not merely those to which an entry isnot to be made. For this reason, in this case operation 2 as set outabove would be divided into two equal intervals. In the first of theseintervals, if the stepper is in a position corresponding to the enteredclass, switch 53 would be closed to increase the charge in thecapacitor. In the second interval, whatever the position of the stepper43, the discharge resistor s would be connected in circuit by switchcontact 55. Other forms of display are possible; for example in FIG. I,by leaving switch contact 35 open, a histogram display is producedrepresenting all entries into the various classes for a predeterminedperiod but in which the vertical deflection scale is exponential.

If it is required to provide a permanent record, a chart recorder may beconnected to the common line 29 instead of or in addition to the cathoderay tube display 37. A sequential operation of the switches 20, 21, 22would then have to be carried out at a rate appropriate to the recorderperformance. This would be normally much slower than the sampling rateand it would be convenient therefore to interrupt the sampling leavingthe switch unit 30 inoperative whilst a record is made on the chartrecorder. This histogram would thus be plotted on the chart, the classwidth axis being provided by the chart traverse. It will be noted thatall the capacitors Ill, 11, 12 etc. are isolated when they are not beingcharged or discharged. The time for which a given voltage distributionon the capacitors can be stored is therefore determined primarily by thequality of the capacitors.

FIG. 3 illustrates a modification of part of the system of FIG. I andmention will only be made of the distinctive features of FIG. 3. It willbe noted that the switch unit 39 of FIG. 1 functions essentially as ifit were a switch having a normal position in which there is an opencircuit but which serves to connect the line 29 either to the constantvoltage source or to the zero potential point. Considering such a switchit will be seen that, instead of using two resistances 31, 32 in the twoalternative circuits, a single resistor might be provided between theswitch contact and the line 29. In FIG. 3 such a switch arrangement isshown but, instead of a single resistor between the switch and the line29, there are shown separate resistors 70, '71, '72 provided for each ofthe capacitors 10, 11, 12 etc. The switch now is a contact arm 7d whichis normally in an open circuit position but can move either to acharging circuit from a charging potential source 75 or to a zeropotential point 76. The use of separate resistors 70, 71, 72 allows thereadout circuit to have a different sensitivity for each class intowhich the magnitudes of the variable are classified. In FIG. 3, toeffect this readout, separate transistors 90, 91, 82 are provided foeeach class. The collector of each transistor is connected to a suitablesource of supply potential. The bases of the various transistors areconnected to the respective junctions between the resistors 71), 71, 72etc. and the switches 20, 21, 22 etc. The emitters of the varioustransistors are connected through resistors 95 to the modulator 39 andthus to the display 37 and also via common load resistor 57 to the line19. It will be seen that, when the various switches 20, 21, 22 areclosed, the outputs remaining in the respective transistors are appliedto the display. The arrangement of FIG. 3 might be used for example ifthe histogram scale is to be weighed according to the class number so asto give a display in which vertical deflections are proportional to(n/N) d where d is the deviation.

FIG. 41 illustrates another modification of the arrangement of FIG. 1 inwhich an auxiliary capacitor 911 is used to transfer charges to and fromthe storage capacitors 10, 11, 12 etc. instead of using the resistivecharging and discharging switch unit 30 As shown in FIG. d the arm of aswitch 91 is connected directly to the common line 29 and this line isconnected to one pole of the capacitor the other pole of which isconnected through a resistor 92 to ground. The switch 91 is normallyopen circuit but can connect the line 29 either to a charging potentialsource 93 or zero potential point 94. With the arrangement of FIG. 9 theswitching sequence has to be slightly different from that of FIG. I inthat the switch 91 has to be closed and opened so as to put theappropriate charge on the auxiliary capacitor 90 before each operationof each one of the switches 20, 21, 22 etc. Thus the cycle of operationis first the closing of one of the contacts of the switch 91 by movingthe arm of the switch either to the lead to the source 93 or the lead tothe zero potential point 941 according to whether or not a charge is tobe added to a storage capacitor 10, 11, 12 etc. The switch arm 91 isthen put in the open circuit position and the appropriate one of theswitches 10, II, 12 is closed and subsequently reopened. This transferscharge from the capacitor 90 to one of the storage capacitors 10, 11, 12or removes part of the charge from one of these storage capacitors bysharing it with auxiliary capacitor. Subsequently the switch arm 91 ismoved to make contact with the lead to the zero potential point 941 soas to discharge the auxiliary capacitor 99 before starting the operationagain for the next one of the storage capacitors. Each time thecapacitor 90 is finally discharged, a pulse is generated across theresistor 92 and this voltage is applied to a line 96 leading. to themodulator 39 to feed the display. In this arrangement the display willhave to be gated so that only the voltage pulse is utilized. The timeconstant of the circuit constituted by the auxiliary capacitor 99 andthe resistor 92 has to be made short compared with the duration ofoperation of the switches 2'11, 21, 22 etc. to ensure complete chargingand discharging of the auxiliary capacitor 90.

lclaim:

I. A statistical display apparatus for displaying updated histograminformation about the magnitude of a variable com prising:

a number of storage capacitors corresponding to the number of classesinto which the magnitude of the variablc is to be classified,

signal amplitude sampling and classifying means for periodicallysampling the variable to determine the instantaneous magnitude of thevariable and for classifying its magnitude into the appropriate one ofthe classes in accordance with the determined magnitude, circuit meanscontrolled by said signal amplitude sampling and classifying means forfeeding a predetermined unit of charge into the capacitor correspondingto the appropriate class each time the sample falls within the class,

means for periodically removing part of the charge on each of thecapacitors and display means for displaying the magnitude of theresultant charges on each of the various capacitors.

2. A statistical display apparatus as claimed in claim 1 wherein saidstorage capacitors are all of equal capacitance and wherein all thecharging and discharging circuits are similar.

3. A statistical display apparatus as claimed in claim 1 wherein saiddisplay means comprises a cathode-ray tube display.

4. A statistical display apparatus as claimed in claim 3 wherein saiddisplay means includes means for scanning the voltages on the variousstorage capacitors in sequence and means for applying the successivelyscanned voltages to effect transverse deflection of a line scan trace tosimulate a histogram display.

5. A statistical display apparatus as claimed in claim 1 whereincharging of the selected capacitor is effected by connecting thecapacitor through a resistance to a constant voltage source for a timeperiod short compared with the time constant of the charging circuit.

6. A statistical display apparatus as claimed in claim 1 whereincharging of the selected capacitor is effected by connecting thecapacitor to a constant current source.

7. A statistical display apparatus as claimed in claim 1 whereincharging of the selected capacitor is effected by connecting it to anauxiliary capacitor which has previously been charged.

8. A statistical display apparatus as claimed in claim wherein thecircuit means controlled by the classifying means for feeding a chargeinto the appropriate capacitor comprises:

a common input/output line,

a series of switches, one for each capacitor, which switches connect thecapacitors to said common input/output line,

a stepper for closing and reopening the switches one at a time insequence, and

means for feeding a charging current to said input/output line when theswitch corresponding to a capacitor on which the charge is to beincreased is closed.

9. A statistical display apparatus as claimed in claim 8 wherein meansare provided for connecting said input/output line to a discharge pointeach time a switch is closed not corresponding to a capacitor on which acharge is to be increased.

10. A statistical display apparatus as claimed in claim 8 wherein saidmeans for feeding a charging current to said input/output line comprisesa capacitor charge source, a capacitor discharge point, switch means forconnecting said input/output line,to said charge source on saiddischarge point, and coincidence circuits for comparing the stepperposition with the classified magnitude of the variable from said meansfor periodically sampling the variable, said coincidence circuit beingarranged, on coincidence of the stepper position with the classifiedmagnitude, to operate said switch means to connect said input/outputline to said charge source but, on noncoincidence, to connect theinput/output line to said discharge point.

11. A statistical display apparatus as claimed in claim 8 wherein saidinput/output line is connected to said display means by a switchoperated to effect the connection only when one of the storage capacitorswitches is closed.

12. A statistical display apparatus as claimed in claim 8 wherein meansare provided for initiating a cycle of operation of the stepper eachtimeghtavagiable is sampled.

1. A statistical display apparatus for displaying updated histograminformation about the magnitude of a variable comprising: a number ofstorage capacitors corresponding to the number of classes into which themagnitude of the variable is to be classified, signal amplitude samplingand classifying means for periodically sampling the variable todetermine the instantaneous magnitude of the variable and forclassifying its magnitude into the appropriate one of the classes inaccordance with the determined magnitude, circuit means controlled bysaid signal amplitude sampling and classifying means for feeding apredetermined unit of charge into the capacitor corresponding to theappropriate class each time the sample falls within the class, means forperiodically removing part of the charge on each of the capacitors anddisplay means for displaying the magnitude of the resultant charges oneach of the various capacitors.
 2. A statistical display apparatus asclaimed in claim 1 wherein said storage capacitors are all of equalcapacitance and wherein all the charging and discharging circuits aresimilar.
 3. A statistical display apparatus as claimed in claim 1wherein said display means comprises a cathode-ray tube display.
 4. Astatistical display apparatus as claimed in claim 3 wherein said displaymeans includes means for scanning the voltages on the various storagecapacitors in sequence and means for applying the successively scannedvoltages to effect transverse deflection of a line scan trace tosimulate a histogram display.
 5. A statistical display apparatus asclaimed in claim 1 wherein charging of the selected capacitor iseffected by connecting the capacitor through a resistance to a constantvoltage source for a time period short compared with the time constantof the charging circuit.
 6. A statistical display apparatus as claimedin claim 1 wherein charging of the selected capacitor is effected byconnecting the capacitor To a constant current source.
 7. A statisticaldisplay apparatus as claimed in claim 1 wherein charging of the selectedcapacitor is effected by connecting it to an auxiliary capacitor whichhas previously been charged.
 8. A statistical display apparatus asclaimed in claim wherein the circuit means controlled by the classifyingmeans for feeding a charge into the appropriate capacitor comprises: acommon input/output line, a series of switches, one for each capacitor,which switches connect the capacitors to said common input/output line,a stepper for closing and reopening the switches one at a time insequence, and means for feeding a charging current to said input/outputline when the switch corresponding to a capacitor on which the charge isto be increased is closed.
 9. A statistical display apparatus as claimedin claim 8 wherein means are provided for connecting said input/outputline to a discharge point each time a switch is closed not correspondingto a capacitor on which a charge is to be increased.
 10. A statisticaldisplay apparatus as claimed in claim 8 wherein said means for feeding acharging current to said input/output line comprises a capacitor chargesource, a capacitor discharge point, switch means for connecting saidinput/output line to said charge source on said discharge point, andcoincidence circuits for comparing the stepper position with theclassified magnitude of the variable from said means for periodicallysampling the variable, said coincidence circuit being arranged, oncoincidence of the stepper position with the classified magnitude, tooperate said switch means to connect said input/output line to saidcharge source but, on noncoincidence, to connect the input/output lineto said discharge point.
 11. A statistical display apparatus as claimedin claim 8 wherein said input/output line is connected to said displaymeans by a switch operated to effect the connection only when one of thestorage capacitor switches is closed.
 12. A statistical displayapparatus as claimed in claim 8 wherein means are provided forinitiating a cycle of operation of the stepper each time the variable issampled.